A three-phase rotating electrical machine including a fractional slot configuration is used for the purpose of reducing torque ripple in a case where the three-phase rotating electric machine is used as an electric motor and approximating a voltage wave to a sine wave in a case where the three-phase rotating electric machine is used as an electric generator. However, at the three-phase rotating electrical machine including the fractional slot configuration, it is likely that noises and/or vibrations occurring during operation increase compared to a three-phase rotating electrical machine including an integral slot configuration. Examples of techniques of reducing the noises and/or vibrations occurring during the operation of the three-phase rotating electrical machine including the fractional slot configuration are disclosed in JP2003-32983A, JP2010-75049A, JP2002-165428A and JP2000-69695A (which will be hereinafter referred to as Patent references 1 to 4, respectively).
According to a known rotating electrical machine of Patent reference 1, the following expression (1) is established between a number Ns of slots of an armature and a number P of magnetic poles of a rotor. The known rotating electrical machine of Patent reference 1 includes a fractional slot configuration. Ns=3×{P/2−INT(P/10)} . . . (1). However, P≠6n or P≠8m (each of n and m is an integer) and INT(P/10) is a value of an integral part of a quotient obtained when P is divided by 10. Further, in an embodiment of Patent reference 1, a configuration in which the number Ns of slots is 18 and the number P of magnetic poles is 14 is described as an example. It is described in Patent reference 1 that the rotating electrical machine with small noises and vibrations and a high output density can be provided.
In Patent references 2 and 3, a rotating electrical machine including a fractional slot configuration is disclosed in which a number Ns of slots and a number P of magnetic poles are limited. An effect of Patent reference 2 is similar to the effect of Patent reference 1. On the other hand, according to Patent reference 3, vibration of low-order harmonics does not occur and slot ripple is reduced to be low.
According to a permanent magnet rotor of Patent reference 4, substantially circular shaped rotor core sheets are stacked to form rotor cores, and permanent magnets are buried within the rotor cores. Long holes are provided in contact with end faces of the permanent magnets, ends of positive polar or ends of negative polar. When an angle with regard to a pole boundary of the long hole is θi, the rotor core sheets include N kinds of rotor core sheets having different θi, and values from θi to θN are specified. According to Patent reference 4, leakage of flux between the rotor magnetic poles is reduced and pseudo skew is provided in a radial direction in a balanced manner. As a result, noises and vibrations that are caused by cogging torque and/or torque ripple can be lowered.
The exemplified techniques of Patent references 1 to 3 are ideal for the use in a limited range of the number of rotations because the techniques can achieve the low torque ripple, and the low noises and vibrations in such a use. For the use in a wide range of the number of rotations, however, the number of magnetic poles of a rotor cannot be increased due to a restriction of control.
In addition, at the three-phase rotating electrical machine including the fractional slot configuration, the lowest order of a spatial oscillation mode of an electromagnetic force generally corresponds to the greatest common divisor of the number Ns of the slots and the number P of the magnetic poles. In Patent reference 3, the number Ns of the slots and the number P of the magnetic poles which prevent the occurrence of the electromagnetic force (a vibratory force) of the spatial oscillation mode of the second order or lower is defined or specified, however, combinations of the numbers Ns of the slots and the numbers P of the magnetic poles which cause the electromagnetic force of the spatial oscillation mode of the third order and the fourth order are also specified. In this case, deformation modes of a stator which are the third order and the fourth order resonate with the electromagnetic force, and thus the low vibrations and the low noises may not be achieved.
The technique of Patent reference 4 is to reduce the noises and vibrations that are caused by the cogging torque and/or torque ripple exclusively in an integral slot configuration. Therefore, the technique of Patent reference 4 cannot be used in the fractional slot configuration.
A need thus exists for a three-phase rotating electrical machine including a fractional slot configuration, which is not susceptible to the drawbacks mentioned above.